1. Field of the Invention
The invention relates generally to exact image reconstruction in a cone beam computed tomography (CT) imaging system, and more specifically to a method and apparatus for reducing radiation exposure to the object being imaged.
2. Description of the Prior Art
Recently a system employing cone beam geometry has been developed for three-dimensional (3D) computed tomography (CT) imaging that includes a cone beam x-ray source and a 2D area detector. An object to be imaged is scanned, preferably over a 360.degree. angular range and along its entire length, by any one of various methods wherein the position of the area detector is fixed relative to the source, and relative rotational and translational movement between the source and object provides the scanning (irradiation of the object by radiation energy). The cone beam approach for 3D CT has the potential to achieve 3D imaging in both medical and industrial applications with improved speed, as well as improved dose utilization when compared with conventional 3D CT apparatus (i.e., a stack of slices approach obtained using parallel or fan beam x-rays).
As a result of the relative movement of the cone beam source to a plurality of source positions (i.e., "views") along the scan path, the detector acquires a corresponding plurality of sequential sets of cone beam projection data (also referred to herein as cone beam data or projection data), each set of cone beam data being representative of x-ray attenuation caused by the object at a respective one of the source positions.
As well known, and fully described for example in the present inventor's U.S. Pat. No. 5,257,183 entitled METHOD AND APPARATUS FOR CONVERTING CONE BEAM X-RAY PROJECTION DATA TO PLANAR INTEGRAL AND RECONSTRUCTING A THREE-DIMENSIONAL COMPUTERIZED TOMOGRAPHY (CT) IMAGE OF AN OBJECT issued Oct. 26, 1993, incorporated by reference herein, image reconstruction processing generally begins by calculating Radon derivative data from the acquired cone beam data. The Radon derivative data is typically determined by calculating line integrals for a plurality of line segments L drawn in the acquired cone beam data. In the embodiment described in detail in the '183 patent, Radon space driven conversion of the derivative data is used to develop an exact image reconstruction of a region of interest (ROI) in the object.
A cone beam data masking technique which improves the efficiency of the calculation of the Radon derivative data in such a Radon space driven technique is described in the present inventor's U.S. Pat. No. 5,504,792 entitled METHOD AND SYSTEM FOR MASKING CONE BEAM PROJECTION DATA GENERATED FROM EITHER A REGION OF INTEREST HELICAL SCAN OR A HELICAL SCAN, issued Apr. 2, 1996, and incorporated by reference herein. The masking technique facilitates efficient 3D CT imaging when only the ROI in the object is to be imaged, as is normally the case. In the preferred embodiment described therein, a scanning trajectory is provided about the object, the trajectory including first and second scanning circles positioned proximate the top and bottom edges, respectively, of the ROI, and a helical scanning path is connected therebetween. The scanning trajectory is then sampled at a plurality of source positions where cone beam energy is emitted toward the ROI. After passing through the ROI, the residual energy at each of the source positions is acquired on a detector as a given one of a plurality of sets of cone beam projection data. Each set of the cone beam projection data is then masked so as to remove a portion of the cone beam projection data that is outside a given sub-section of a projection of the ROI in the object and to retain cone beam projection data that is within the given sub-section. The masked cone beam projection data is then processed so as to develop reconstruction data, and an exact image of the ROI is developed by combining the reconstruction data. Hence, the masks are commonly referred to as "datacombination" masks.
Data-combination masks can also be used to improve the efficiency of the calculation of the derivative data in a detector data driven technique, such as the 3D backprojection technique described in the present inventor's U.S. Pat. No. 5,881,123 entitled SIMPLIFIED CONE BEAM IMAGE RECONSTRUCTION USING 3D BACKPROJECTION, issued Mar. 9, 1999, also incorporated herein by reference.
The present inventor's U.S. patent application Ser. No. 09/274,189 entitled EXACT REGION OF INTEREST CONE BEAM WITHOUT CIRCLE SCANS, filed Mar. 22, 1999, incorporated by reference herein, improved upon the invention described in the forenoted U.S. Pat. No. 5,504,792, by providing an exact image reconstruction of an ROI in an object without the requirement that the source scan path have top and bottom circle scan path trajectories proximate the top and bottom edges of the ROI. Furthermore, the improvement is applicable to both of the Radon space and detector driven types of image reconstruction processing. As described in this U.S. patent application Ser. No. 09/274,189, and consistent with the techniques described in the above noted U.S. Pat. Nos. 5,881,123 and 5,504,792, when calculating the derivative data, the length of the line segments L formed in the acquired cone beam data are determined by the boundaries of the data combination mask. However, when processing line segments L formed in cone beam data acquired at source positions near the top or bottom edges of the ROI, groups of the line segments L have one of their end points determined by a horizontal line (the x-axis) of the mask. Acquired cone beam data which resides on one side of the horizontal axis of the mask is not used. Thereafter, integral data calculated for the line segments L formed in the masked cone beam data are processed so as to develop contribution to a 3D image reconstruction of the ROI in the object. Since some of the acquired projection data is not used, some of the radiation exposure suffered by the object is unnecessary. This is undesirable, especially if the object is a human being.
It would be desirable to provide a method and apparatus for exact image reconstruction processing which makes more efficient use of the X-ray dose applied to the object being imaged.